Comparison of the monocular occlusion and a direct method for objective measurement of fixation disparity.

PURPOSE

Previous studies have measured objective fixation disparity using a monocular occlusion method, by which the monocular components of misalignment are revealed by measuring the shift in the position of the fixating eye after occlusion of the other eye.

METHODS

We assessed the use of the monocular occlusion method by comparing it to direct measurements of binocular vergence.

RESULTS

The direct method required precise calibration, but was found to be reliable. Several problems were discovered with the monocular occlusion method: (1) when using a central fusion stimulus, fixation was difficult to control accurately enough to measure the small monocular shifts in eye position expected at lower vergence demands; (2) with a peripheral fusion target, objective fixation disparity could not be measured directly; and (3) upon occlusion, a variable saccade occurred in the fixating eye.

CONCLUSION

Objective measurement of fixation disparity with the direct method is quicker and more reliable than with the monocular occlusion method.